Remove StreamableReader

It's use tempted the creation of PROTOTYPEs. The only classes that
legitimately implement a readFrom method are those extending from
Diffable - such behavior is part of cluster state management and
out of scope for the PROTOTYPE cleanup.

core/src/main/java/org/elasticsearch/common/io/stream/StreamableReader.java

@@ -1,33 +0,0 @@
-/*
- * Licensed to Elasticsearch under one or more contributor
- * license agreements. See the NOTICE file distributed with
- * this work for additional information regarding copyright
- * ownership. Elasticsearch licenses this file to you under
- * the Apache License, Version 2.0 (the "License"); you may
- * not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *    http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing,
- * software distributed under the License is distributed on an
- * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
- * KIND, either express or implied.  See the License for the
- * specific language governing permissions and limitations
- * under the License.
- */
-package org.elasticsearch.common.io.stream;
-
-import java.io.IOException;
-
-/**
- * Implementers can be read from {@linkplain StreamInput} by calling their {@link #readFrom(StreamInput)} method.
- *
- * Implementers of this interface that also implement {@link Writeable} should see advice there on how to do so.
- */
-public interface StreamableReader<T> {
-    /**
-     * Reads an object of this type from the provided {@linkplain StreamInput}. The receiving instance remains unchanged.
-     */
-    T readFrom(StreamInput in) throws IOException;
-}